The study will enroll pregnant or postpartum HIV-1-infected women, and their HIV-uninfected infants in Botswana. At 2-4 weeks of age, live HIV-uninfected infants will be randomized to receive either double-blinded cotrimoxazole (CTX) or placebo from 2-4 weeks through 15 months. In addition, breastfeeding (BF) infants will be randomized to BF until either 6 or 12 months of age. Children will be followed prospectively until 18 months of age. The primary endpoint will be survival at 18 months comparing all infants in the CTX vs. placebo arms, and by randomized duration of BF among those BF at randomization. Secondary endpoints will evaluate survival and morbidity/mortality at 12 and 15 months; HIV-free survival to 18 months; and the safety of CTX prophylaxis. Secondary observational objectives include comparing MTCT and mortality by initial feeding method (formula feeding or any BF > 1 month), and an analysis of maternal characteristics as predictors for initial feeding choice and HIV-free survival. All women and infants will receive standard antenatal and peripartum prophylaxis from the Botswana government for MTCT prevention (PMTCT), and will choose a feeding method with counseling. Breastfeeding infants will receive infant nevirapine (NVP) prophylaxis or will be protected from MTCT by the use of maternal HAART.